Ultrasonic rangefinder

ABSTRACT

An ultrasonic rangefinder capable of detecting a distance between a vehicle and an object with high accuracy is shown. The measure of microphones for transmitting and receiving ultrasonic waves is chosen so that detour waves are restricted.

This application is a division of application Ser. No. 737,937, filedMay 28, 1985 now U.S. Pat. No. 4,739,860.

BACKGROUND OF THE INVENTION

The present invention relates to an ultrasonic rangefinder which is usedto transmit and receive an ultrasonic wave signal for the detection ofthe distance from an object and, in particular, to improvements of anultrasonic rangefinder for preventing a detour wave from reducing theaccuracy of the detection.

There are already several ultrasonic rangefinders in existence, forexample the one described in Utility Model Published Application No. Sho57-68574. Such a device emits ultrasonic wave from an ultrasonic wavetransmitter and receives pulses reflected from an object by anultrasonic wave receiver. The distance is found from the phasedifference between the emitted pulse and the received pulse.

As technically discussed in the art, received waves on a receiverinclude a detour wave which reduce the accuracy of detection. In arangefinder, particularly in a rangefinder which detects the vehiclehight (the distance from the undersurface of the body of the vehicle tothe ground), as disclosed in Japanese Patent Published Application No.Sho 57-182544, the reflected wave and the detour wave partially overlap,because the detected distance is short, (15 to 40 cm).

In a case of this type, the overlapping portions mutually interfere,causing the signal level to vary. In addition, when using therangefinder on snow-covered roads and sandy soil, giving fullconsideration to the fact that the reflected wave signal is diminishedbecause the ultrasonic wave signal is absorbed, there is a greatnecessity to set a low threshold level, and for this reason it isespecially easy to produce the abovementioned error.

It has been proposed, as shown in Japanese Patent Published ApplicationNo. Sho 53-21953, to provide horns on a wave transmitter and a wavereceiver for improving the directionality of the wave transmitter andreceiver.

However, even with the improvement in directionality provided by thehorns in such an ultrasonic rangefinder, when an ultrasonic transmitterand an ultrasonic receiver are installed in close proximity, a detourwave of an appreciable level which is directly incident from the wavetransmitter and not reflected from the target object, is received inaddition to a reflected wave.

This detour wave especially when an object at a short distance is beingdetected (such as in the case of gauging the height of an automobile, aspreviously mentioned) partially overlaps the reflected wave and createsan obstruction, and is apt to cause a detection error by mixing with thereflected wave.

Furthermore, in a low temperature ambience the type of insulatorsnormally used for damping the detour wave which transmits machanicallyfrom the transmitter to the receiver inside the rangefinder suffer adecrease in elasticity, and because their noise prevention effectivenessdeteriortes, the level of the detour wave becomes higher than when atnormal temperatures.

SUMMARY OF THE INVENTION

An object of the present invention is to provide, with due considerationto the drawbacks of such conventional devices, an ultrasonic rangefinderin which the detour wave is effectively damped.

Further object of the present invention is to improve the detectionperformance of an ultrasonic rangefinder by effectively damping thedetour wave through an improvement in the shape or positions oftransmitting and/or receiving port of the rangefinder.

A still further object of the present invention is to provide anultrasonic rangefinder which can prevent the effect of the detour waveeven in a low temperature ambience.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will become more apparent from the following description ofpreferred embodiments taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a block diagram showing an example of the electricalconfiguration of an ultrasonic rangefinder.

FIG. 2A and FIG. 2B are waveform diagrams showing transmitted andreceived signals of a conventional rangefinder.

FIG. 3 is a sectional view showing the inside of a conventionalultrasonic rangefinder.

FIG. 4A and FIG. 4B are waveform diagrams showing a transmitting waveand the generated status of the detour wave.

FIGS. 5A to 5C and FIGS. 6A to 6C are waveform diagrams showing thewaveforms of the detour wave and reflected waves when they overlap, andthe time difference change.

FIG. 7 is a sectional view showing the configuration of a firstembodiment of the present invention.

FIG. 8 is a graph showing the relationship between the horn aperturediameter and S/N of an ultrasonic rangefinder, obtained by actual test.

FIG. 9 is a graph showing the relationship between the length of thestraight tubular section of the horn and S/N, obtained by actual test.

FIG. 10 is a graph showing the relationship between the radius of thewave transmitting/receiving surface and the optimum length of thestraight tubular section, obtained by actual test.

FIG. 11 is a graph showing the results of actual tests to determine therelationship between the wave length of the ultrasonic wave signal andthe optimum length of the straight tubular section.

FIG. 12 is a diagram of the directional characteristics of the wavetransmitting means of the first embodiment of the present invention.

FIG. 13 is a diagram of the directional characteristics of the wavereceiving means of the first embodiment of the present invention.

FIG. 14 is a sectional view showing the configuration of a secondembodiment of the present invention.

FIG. 15 is a plan view showing the upper surface of the secondembodiment of the present invention.

FIG. 16 is a sectional view showing the configuration of a thirdembodiment of the present invention.

FIG. 17 is a sectional view showing the configuration of a fourthembodiment of the present invention.

FIG. 18 is a characteristic diagram for the fourth embodiment of thepresent invention, showing the change in rigidity versus temperature forinsulators housing transmitting and receiving means.

FIG. 19 is a characteristic diagram for the fourth embodiment of thepresent invention showing the effect of damping on the detour waveversus change in temperature.

FIG. 20 is a sectional view showing the configuration of a fifthembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To facilitate the understanding of the present invention, theconventional ultrasonic rangefinder will be briefly discussed.

The configuration of the electrical components of such an ultrasonicrangefinder is shown in FIG. 1.

A blocking oscillator 1, as shown in FIG. 1, generates a high frequencysignal S₁ for a specified time interval t₁, every period t₂, and supliesan ultrasonic wave transmitting means 2. As a result, an ultrasonic wavepulse signal from the ultrasonic wave transmitting means 2 istransmitted toward a target object D.

Then, the ultrasonic wave pulse signal relfected from the target objectD enters an ultrasonic wave receiving means 3, upon which the receivedsignal is amplified in an amplifier 4, then the amplified signal S₂ isinput into a time difference measuring curcuit 5.

This time difference measuring circuit 5 determines the time interval t₃(shown in FIG. 2B) from the starting point of the high frequency wavesignal S₁ output from the blocking oscillator 1, to the starting pointof the previously mentioned received signal S₂ after amplification, andtreats this as data giving the distance to the target object.

The starting point of the received signal S₂ is set as the point wherethe level of the received signal S₂ exceeds a specified threshold levelA.

The ultrasonic wave transmitting means 2 and the ultrasonic wavereceiving means 3, as shown in FIG. 3, are housed in a common casing 8.

The casing 8 is formed from a plastic such as a synthetic resin. As wellas housing the ultrasonic wave transmitting means 2 and the receivingmeans 3, this casing 8 is formed to provide a transmitting port K₂ and areceiving port K₃, both of which are horn-shaped, and which lead to atransmitting surface 2a and a receiving surface 3a, respectively.

The detour wave includes a wave which transmits sonically inside therangefinder from the transmitting means 2 to the receiving means 3 aswell as a wave which propagates and is detoured through the air from thetransmitting part K₂ to the receiving port K₃.

A plurality of insulators 7A and 7B, made from rubber or some suchelastic material, are interposed between the casing 8 and thetransmitting and receiving means 2 and 3 for prevention transmittingsignals from entering the receiving means inside the casing 8.

As shown in FIG. 3, when the ultrasonic wave transmiting means 2 and theultrasonic wave receiving means 3, are in close proximity and are housedin the casing 8, as shown in FIG. 4B, the vibrations from the wavetransmitting means are transmitted into the casing 8 as the signalreceived in the receiving means 3 in addition to the reflected wavesignal S₂ from the target object D, and the reception level of thedetour wave S₃ arriving at the wave receiving means 3 cannot be ignored.

In particular, the reflected wave S₂ and the detour wave S₃ partiallyoverlap in equipment to detect the vehicle height (the distance from theundersurface of the body of the vehicle to the ground), because thedetected distance is short, (15 to 40 cm).

In a case of this type, the overlapping portions mutually interfere,causing the signal level to vary.

Furthermore, the pulse width t₃ of the detour wave S₃ is distortedaccording to the route by which it reached the wave receiving means 3from the transmissing means 2.

Specifically, in the case where the detour wave S₃ and the reflectedwave S₂ are received in the same phase (shown in FIG. 5A), the levels oftheir component waves are added, as shown in FIG. 5B, and the startingpoint of the reflected wave S₂ (the point where the threshold level A isexceeded) advances (by Δt' as shown in FIG. 5C).

When the signals are received as opposite phases (shown in FIG. 6A), thelevels of their component waves are subtracted, as shown in FIG. 6B),and the starting point of the reflected wave S₂ is retarded (by Δt" asshown in FIG. 6C).

Accordingly, at this sort of a starting position for the reflected waveS₂, there is an overlapping, and, as shown in FIG. 5C and FIG. 6F, anerror range is produced for t' or t" in the time difference t₃, and theaccuracy of detection diminishes.

Hereinafter some embodiments of the present invention will be describedwith the accompanied drawings in which same references are given forsame parts as in FIGS. 1 and 3.

A first embodiment of the present invention is shown in FIG. 7.Referring to the figure, ultrasonic wave transmitting means 2 and anultrasonic wave transmitting means 3 are housed side by side in ahousing chamber of a casing 10, which is formed of a synthetic resinsuch as a plastic.

A plurality of insulators 11 and 12, which are made from a materialwhich effectively insulates sound, such as a flexible rubber, isinterposed between the casing 10 and the transmitting and receivingmeans 2 and 3, with the exception of a transmitting surface 2a and areceiving surface 3a.

In the bottom surface of the casing 10 are provided a plurality of horns13 and 13 which are formed as orifices communicating with thetransmitting surface 2a and the receiving surface 3a respectively.

The horns 13 and 14 comprise straight tubular sections 13a and 14a,formed in a cylindrical shape pointing in a downward direction from theperipheries of the transmitting surface 2a and the receiving surface 3arespectively, and conical sections 13b and 14b, formed in a cone shape,opening in the downward direction from the bottom edges of the straighttubular sections 13a and 14a toward the bottom surface of the casing 10.

The length x of the straight tubular sections 13a and 14a is set tosatisfy the relationship.

    x=1.5 r.sup.2 /λ                                    (1)

where r is the radius of the transmitting and receiving surfaces 2a and3a, and λ is the wave length of the ultrasonic signal produced by thewave transmitting means 3.

Also, the aperture diameter y of the horns 13, 14 is set to that

    y=2λ                                                (2)

By shaping the horns 13, 14 so that they satisfy the relationships shownin the above equation (1) and (2), the detour wave is considerablydamped, and it is possible to avoid its effect.

The results of tests performed by the inventors of the present inventionto derive the above equations (1) and (2) are given below.

The wave transmitting and receiving means 2, 3 used in these tests havea center frequency of 40 KHz (wave length λ=8.5 mm), and the radius r ofthe transmitting and receiving surfaces 2a, 3a is 5.4 mm. (N.B. Becausethe normal oscillation mode of the transmitting and receiving means isbending mode, here the effective radius of the vibrating surfaces isconsidered to be 5.0 mm).

The test results shown in FIG. 8 give the variation in S/N forvariations in the aperture diameter y of the horns 13, 14 (where S/N isthe comparative strength of the reflected wave and the detour wave withan aluminum plate placed 30 cm away from the aperture surface).

From the same drawing it can be determined that S/N is a maximum whenthe aperture diameter y is 18.5 mm (which is about double the wavelength λ).

The test results shown in FIG. 9 give the S/N ratio for different valuesof x, the length of the straight tubular section, with the horns 13, 14having an aperture diameter of 18.5 mm, which is determined to be theoptimum diameter from the test results of FIG. 8.

From the same drawing it can be determined that S/N is a maximum whenthe length of the straight tubular section x is approximately 4.5 mm.

Next, considering the radius r of the transmitting and receivingsurfaces and the wave length of the ultrasonic wave signal as parametersrelated to the optimum value of the length x of the straight tubularsection, the following expression is obtained.

    x=f (r, λ)                                          (3)

The test results shown in FIG. 10 give the length x of the straighttubular section with S/N at a maximum (which is the optimum straighttubular section length) when the radius r of the transmitting andreceiving surfaces is varied with the wave length λ at a fixed value(8.5 mm).

The test results shown in FIG. 11 give the length x of the straighttubular section when the wave length λ is varied while holding theradius r at a fixed value (5 mm).

From these two drawings it can be determined that the optimum length xof the straight tubular section is proportional to the square of theradius and is inversely proportional to the wave length λ.

Accordingly, equation (3) may be expressed as

    x=k·r.sup.2 /λ                             (4)

(where k is a constant).

Here, if the optimum length of the straight tubular section, determinedfrom the test results shown in FIG. 9 as x=4.5 mm, and λ=8.5 (mm), r=5(mm) are substituted in equation (4), the value of the constant k iscalculated as k=1.5 and the equation (1) are derived.

Similarly, the optimum value of the length x of the straight tubularsections 13a, 14a can be obtained from equation (1) for diverse valuesof λ and r.

The results of the tests on the damping of the detour wave, provided bythe shape of the horns, are shown in FIG. 12 and FIG. 13. FIG. 12 showsthe directional characteristics of the wave transmitting means 2, whileFIG. 13 shows the directional characteristics of the wave receivingmeans 3. In the diagram, P indicates the characteristics of the presentinvention, and Q indicates the characteristics of a means not using thehorns.

From these two diagrams, in the case of this embodiment of the presentinvention, the amount of damping from the means with the horns is largein a lateral side. Specifically it is determined that adequate dampingof the detour wave is carried out.

Further, in this embodiment, the provison of the insulators 11, 12results in the noise eliminating effect. Also, by housing the wavetrasmitting and receiving means 2, 3, the positioning error as a resultof the elasticity of the insulators 11. 12 is controlled by the casing.

Further, in the present invention, as shown in FIG. 7, the wavetransmitting and receiving means 2, 3 can obviously be applied for arangefinder having separate casings side by side.

As has been explained in detail above, in the present invention, thedetour wave can be extensively damped by improving the shape of thehorn, and the creation of an obstacle to the detection of the reflectedwave can be prevented.

In addition, the high efficiency of the horn in damping the detour wavemakes it possible to install the wave transmitting and receiving meansin close proximity, so that the ultrasonic rangefinder itself can bereduced in size.

FIG. 14 and FIG. 15 show a second embodiment of the present invention. Acasing 20, formed from synthetic resin such as a plastic, comprises awave transmitting means housing section 20A which houses a wavetransmission means 2, a wave receiving means housing section 20B whichhouses a wave receiving means 3, a connecting section 20C which connectsthe housing sections 20a and 20B, and a circuit housing section 20D.

A signal transmitting port K₂, which transmits an ultrasonic signalproduced by the wave transmitting means 2, is formed in the wavetransmitting means housing section 20A, and a signal receiving port K₃,which introduces the ultrasonic signal of the reflected wave, is formedin the wave receiving means housing section 20B. The aperture surfacesof the signal transmitting port K₂ and the signal receiving port K₃ arepositioned at the same height.

A signal-processing circuit 21 is housed in the connecting section 20C.In addition, a concave section 20E is provided which is formed so thatits surface is more concave than the aperture surfaces of either thesignal transmitting port K₂ or the signal receiving port K₃. The bottomsurface and wall surfaces of the concave section 20E are formed roughlyat right angles.

A signal-processing circuit 23 is housed in the signal housing section20D. In addition, the entire upper surface of the casing 20 is coveredwith a lid 24, and around its circumference this lid 24 is sealed to thecircumferential edge of the casing 20.

A rubber or synthetic plastic insulator 22 is interposed between theside circumference and top surfaces of both the wave transmitting means2 and the receiving means 3.

FIG. 15 is a plan view showing the second embodiment of the presentinvention.

As a result of this type of configuration, in this embodiment of thepresent invention, through the separation of the signal transmittingport K₂ and the signal receiving port K₃ by the connecting section 20C,together with the utilization of the concave section 20E, damping of thedetour wave is achieved and it is possible to prevent the detour wavefrom entering the receiving means 3.

The reason that the concave section 20E damps the detour wave is becausedamping occurs in a plurality of angular sections 20F and 20G, which areformed at the bottom edge of the wall surface of the concave section20E, as a result of the dispersion of the detour wave. The damping whichresults from this dispersion is large, and first occurs at the angularsection 20F of the signal transmitting port K₂, then at the angularsection 20G of the signal receiving port K₃.

A damping effect is also produced by the interference which occurs whenthe detour wave enters the concave section 20E.

Next, a third embodiment of the present invention is shown in FIG. 16.In the same way as in the second embodiment, a casing 30 is providedwith a connecting section 30C, a wave receiving means housing section30B, and a circuit housing section 30D, in addition to a wavetransmitting means housing section 30A, formed as the signaltransmitting port K₂, which has a aperture surface at a different heightthan the signal receiving port K₃.

A concave section 30E is formed at the bottom surface of the connectingsection 30C in the same way as in the second embodiment.

As a result of this type of configuration, in this embodiment of thepresent invention, the same effect is obtained as for the secondembodiment. In addition, as a result of the heights of the signaltransmitting port K₂ and the signal receiving port K₃ being different,it is possible to prevent the detour wave from entering the receivingmeans 3 through diffraction, and is possible to obtain an even greaterdamping of the detour wave.

In this case, it is desirable to position the aperture surface of thesignal receiving port K₃ behind the aperture surfce of the signaltransmitting port K₂ (above, in the drawing). It is also acceptable toeliminate the concave section 30E shown in FIG. 16, and position thebottom surface of the connecting section 30C at the same height as theaperture surface of the signal receiving port K₃. Alternately, thebottom surface of the connecting section 30C may be positioned at aheight midway between the aperture surface of the signal transmittingport K₂ and the aperture surface of the signal receiving port K₃.

As outlined in the foregoing detailed explanation, in the first, secondand third embodiments of the present invention it is possible to dampthe detour signal which is produced by the ultrasonic wave signalgenerated from the wave transmitting means, passing through space toenter the receivng means, and to prevent the detour signal from receivedin the receiving means, so that an accurate detection action may becarried out.

Reffering now to FIG. 17 a fourth embodiment of the present invention isshown.

This embodiment has been conceived for damping vibrations which aretransmitted sonically from a transmittaing means to a receiving meansthrough an inside of the device.

In the ultrasonic rangefinder of this embodiment of the presentinvention, a plurality of insulators 37 and 37 cover the vibratingsurfaces of an ultrasonic wave transmitting means 2 and an ultrasonicwave receiving means 3 other than a wave transmitting surface 2a and awave receiving surface 2b. These insulators 37 and 37 are interposedbetween a casing 8 and the ultrasonic wave transmitting and receivingmeans 2 and 3.

The material of which the insulators 37 and 37 are constructed is afoamed elastic body formed of independent air bubbles on a substrate ofan ethylene-propylene rubber such as ethylene-propylene terpolymer(EPDM) or thylene-propylene copolymer (EPM).

The insulators 37A and 37B made of this kind of material, as shown inFIG. 18, exhibit a smaller change in hardness with change in temperaturethan insulators formed of an unfoamed ethylene-propylene rubber. Even atlow temperatures they do not lose their noise prevention effectiveness.In FIG. 18, the characteristics of the insulators 37A and 37B of thepresent invention are shown as I₁, while the characteristics ofinsulators made of unformed ethylene-propylene rubber are shown as I₂.The comparative modulus temperature TN is a temperature at which therigidity becomes the N multiple.

FIG. 19 is a graph showing the relationship between the temperature andS/N, the ratio of the peak level of the reflected wave S₂ to the peaklevel of the detour wave S₃. The characteristics of this embodiment ofthe present invention are designated M₁ and those of the conventionalmeans shown in FIG. 3, M₂.

From this graph it can be concluded that the damping effect of detourwave in this embodiment is large, and at low temperatures, as well, thiseffect is not diminished.

Accordingly, in this embodiment of the present invention a good dampingeffect on the detour wave S₃ can be obtained, and the reflected wave S₂is always detected with good accuracy over a wide temperature range(-30° to +80° C.).

In addition, the abovementioned ethylene-propylene rubber shows goodresistance to weather and to chemicals, and is suitable for automobileparts.

The foamed elastic body used for the insulators 37A and 37B, because ofits independent air bubble structure, does not become soaked with waterby permeation when used in an environment where it is exposed torainwater, unlike a continuously foamed material, and thus lowperformance because of freezing is prevented.

The foaming ratio of the insulators 37A and 37B is chosen at a ratherhigher value, about 20%, than foaming materials which are broadly used.In addition by use of an agent having small cold hardening it ispossible to reduce still more the hardening of the insulators at lowtemperatures.

A fifth embodiment of the present invention, shown in FIG. 20, will nowbe explained.

In this embodiment, an insulator 37, which covers the ultrasonic wavetransmitting and receiving means 3 and 2 are formed as one integralunit.

The material of construction of this insulator 37 is identical to thatused in the fourth embodiment of the present invention shown in FIG. 17.

By this type of construction, the same effect is obtained as with thefourth embodiment, and, in addition, it is possible to reduce theassembly process and eliminate the positioning variance of thetransmitting means 2 and receiving means 3.

In both fourth and fifth embodiments of the present inventionethylene-propylene rubber is used as the substrate for the insulators,but other materials having a noise deadening effect, such as urethanerubber, are also suitable for this application.

Herein before, several embodiments are described which show severalstructural features. Any structural feature of one embodiment can beapplied for another embodiment so that the combination take a moreeffective configuration against an obstraction due to detour waves.

It should be understood, of course, that the foregoing relates only topreferred embodiments of the present invention and that numerousmodifications or alterations may be made therein without departing fromthe spirit and scope of the invention as set forth in the appendedclaims.

What is claimed is:
 1. An ultrasonic rangefinder for measuring thedistance between the rangefinder and an object, said rangefindercomprising:means for transmitting an ultrasonic wave toward the objectand having a transmitting surface; means for receiving an ultrasonicwave reflected from the object and having a receiving surface; a casinghaving two cavities for housing said transmitting and receiving means,respectively, and provided with a signal-processing unit for processingsignals supplied from the transmitting and receiving means to measurethe distance between the rangefinder and the object, saidsignal-processing unit being disposed in a concave portion formed in aconnecting section between the transmitting and receiving means; and afoaming insulator having an independent cell structure and covering theinterior walls of at least one of said cavities whereby ultrasonic wavesfrom the transmitting means are prevented from directly propagating tothe receiving means through the interior of the casing.